1. Field of the Invention
This invention relates to mounting implantable transducers for use in a hearing aid system within the middle ear.
2. Description of Related Art
In a patient with normally functioning anatomical hearing structures, sound waves are directed into an ear canal by the outer ear and into contact with a tympanic membrane. The tympanic membrane is located at the terminus of the ear canal. The pressure of the sound waves vibrates the tympanic membrane resulting in the conversion to mechanical energy. This mechanical energy is communicated through the middle ear to the inner ear by a series of bones located in the middle ear region. These bones of the middle ear are generally referred to as the ossicular chain, which includes three primary components, the malleus, the incus and the stapes. These three bones must be in functional contact in order for the mechanical energy derived from the vibration of the tympanic membrane to be transferred through the middle ear to the inner ear.
In a patient possessing normal hearing capacity, the tympanic vibrations are mechanically conducted through the malleus, incus, and stapes to the oval window and then into the fluid in the cochlea of the inner ear. Within the cochlea, the mechanical vibrations generate fluidic motion. This fluidic motion is converted into neural impulses and the brain interprets these impulses and derives the patient's perception of sound. A variety of disorders, however, can disrupt or impair normal hearing. These disorders include disorders of the tympanic membrane as well as disorders of the ossicular chain and/or inner ear.
Implantable devices are often useful for assisting with hearing. Such devices include partial middle ear implantable (P-MEI) or total middle ear implantable (T-MEI) devices, cochlear implants, and other hearing assistance systems that use components disposed in the middle or inner ear regions. These components may include an input transducer for receiving sound vibrations or an output stimulator for providing mechanical or electrical output stimuli based on the received sound vibrations.
The cochlear implant, for instance, is an electronic device that allows profoundly deaf people to hear by electrical stimulation of the auditory nerve fibers within the inner ear. Typically, an external microphone will transpond sound waves into electrical energy. A processor will amplify the electrical energy, filter it, and send it to a transmitter which changes the electrical signals into magnetic signals. An implanted receiver transcutaneously senses the magnetic currents, transforms it to an electrical signal, which travels to the cochlea via a wire electrode. This electrode directly stimulates nerve fibers present in the cochlea. The brain perceives this stimulation as sound (see also U.S. Pat. No. 3,764,748).
Some types of partial middle ear implantable (P-MEI), total middle ear implantable (T-MEI), cochlear implant, or other hearing assistance systems utilize components disposed within the middle ear or inner ear regions. Such components may include an input transducer for receiving sound vibrations or an output stimulator for providing mechanical or electrical output stimuli based on the received sound vibrations.
An example of one such device is disclosed in U.S. Pat. No. 4,729,366, issued to D. W. Schaefer on Mar. 8, 1988. In the '366 patent, a mechanical-to-electrical piezoelectric input transducer is associated with a malleus bone in the patient's middle ear. The malleus vibrates in response to sounds received at the patient's tympanic membrane (eardrum). The piezoelectric input transducer transduces a mechanical energy of the malleus vibrations into an electrical signal, which is amplified and further processed by an electronics unit. A resulting electrical signal is provided to an electrical-to-mechanical piezoelectric output transducer that generates a mechanical vibration that is coupled to a stapes bone in the ossicular chain or to an oval window or round window of a cochlea. In the '366 patent, the ossicular chain is interrupted by removal of an incus bone. Removal of the incus prevents the mechanical vibrations delivered by the piezoelectric output transducer from mechanically feeding back to the piezoelectric input transducer.
Piezoelectric transducers are one example of a class of electromechanical transducers that require contact to sense or provide mechanical vibrations. For example, the piezoelectric input transducer in the '366 patent contacts the malleus for detecting mechanical vibrations. In another example, the piezoelectric output transducer in the '366 patent contacts a stapes bone or the oval or round window of the cochlea.
Devices for assisting the hearing impaired patient range from miniaturized electronic hearing devices which can be adapted to be placed entirely within the auditory canal, or implantable devices which can be completely or partially implanted within the skull. For those hearing systems, or portions of hearing systems, that require complete subcranial implantation, a challenge has existed to adapt the implantable device for optimal mounting to the unique patient morphologies (including both naturally occurring as well as those created by surgical processes) among patients. The access site for accessing the implantation area for hearing systems is normally posterior to the flap (or pinna) of the outer ear. The precise morphology of the implantation area of any given patient is normally not determinable until surgical entry into the implantation area is achieved. Thus, it is difficult to fabricate a device that will operably fit within the implantation area prior to surgically accessing the implantation site.
Known implantable devices that have elements which perform a support or mounting function are typically rigidly mounted to a bone within the middle ear region. However, once such systems are positioned and mounted, the devices are not removable from the implantation area without disengaging the support device and any attached apparatus from the bone. As can be readily appreciated removal of previously mounted supporting brackets from tissue and bone creates undesirable trauma as well as stripping of the bone screw holes rendering the holes nearly useless if remounting is necessary.
Further difficulties have arisen with the use of implantable devices in facilitating the fine adjustments necessary to properly position and configure the support assembly and attached transducers so as to contact an auditory element and thus vibrate a portion of the ossicular chain, e.g., the stapes. Such devices present a particular problem in that positioning, or docking, of the transducer against the auditory element in a stable configuration requires extremely fine adjustments that are difficult given the location of the auditory elements and the attendant lack of maneuvering room.